The invention relates to invasive devices which are intended to cooperate with a magnetic resonance imaging apparatus. More particularly, the invention relates to an RF-safe invasive device arranged to be introduced into an object, where the invasive device is elongate shaped and contains at least one long conductor arranged in length to approach or exceed half a wavelength of the RF field generated in the object by operation of the magnetic resonance imaging apparatus.
Conventional invasive devices which include a conductor for cooperating with magnetic resonance imaging (MRI) apparatus are known in which, during RF transmission of the magnetic resonance apparatus, a standing RF wave may be generated within and in the neighborhood of the conductor, considerably. The standing wave may cause dangerous heating of the device and surrounding tissue. More particularly, the conventional arts teach two main groups of invasive devices incorporating long conductors. A first is exemplified by U.S. Pat. No. 5,318,025 (the ""025 patent), incorporated herein by reference.
The ""025 patent discloses an invasive device for use in interventional procedures, where the invasive device is guided to an organ via an opening in the body of a patient. To this end, RF fields and magnetic fields are applied so as to generate magnetic resonance signals of spins in a part of the body (tissue) where a distal end of the invasive device is situated. An RF micro (xcexc)-coil attached to the distal end receives a magnetic resonance signal of spins in a part of the body in the vicinity of the RF xcexc-coil. The signal received by the RF xcexc-coil is transferred to the proximal end by means of an electrical connection and used to determine, in cooperation with the magnetic resonance apparatus, the position of the invasive device in the body of the patient (object) under test.
An example from the second group is PCT Patent No. WO 97/19362 (the ""362 patent), commonly owned with the owner of the instant application, and incorporated herein by reference. The ""362 patent discloses an MR system for interventional procedures, comprising an MR device and a catheter. The MR device is arranged to acquire images of part of an object. A particular portion of the catheter can be imaged in an MR image by providing a conductor loop which comprises two non-magnetic conductors situated at some distance from one another underneath the surface of the catheter. The conductor loop extends along the entire length of the catheter. By application and adjustment of a current through the loop, the homogeneity of the magnetic field of the MR device is affected with the result that the catheter shows up in the image as a dark trace. Thus it is possible to localize the catheter in the image.
Also known is U.S. Pat. No. 5,916,162 (the ""162 patent), commonly owned with the owner of the instant application and incorporated herein by reference. The ""162 patent discloses an invasive device for use in a magnetic resonance imaging apparatus and a means to minimize heating at and around the long conductor incorporated in the device. The ""162 patented invasive device is provided with a hollow carrier which has a distal end and a proximal end, the electric connection extending through the carrier. The electrical connection is provided with an electrically conductive shield which extends over a distance from the distal end to the proximal end and has an electric resistance over said distance which is substantially higher than the electric resistance of the electric connection over the same distance. The electric shield comprises alternately electrically highly conductive portions and electrically less conductive portions.
The distance between the successive electrically less conductive portions is smaller than or equal to approximately xc2xc wavelength of the RF field generated in the object by the magnetic resonance imaging apparatus in order to excite spins for imaging. The shield portions of lower electric conductivity in this case minimize the appearance of standing EM waves in the electric shield, which tends to reduce the development of heat in the electric shield (and the object) because the current induced in the electric shield is substantially smaller than the current which would be induced in the non-shielded connection.
The ""162 patented invasive device, however, is not without drawbacks. For example, numerical estimations show that starting from a catheter with standard diameter a sufficient reduction of heat generation would require a shield of impracticable thickness, and keeping the diameter of a standard catheter fixed and reserving a sensible fraction of that diameter for its original functionality the shield would be too thin to be effective for heat reduction. Such a concept, therefore, is difficult to realize in practice, especially for small diameter catheters such as those used in cardiac- or stroke-related applications.
It is therefore an object of the invention to provide an RF-safe invasive device for introduction into an object being imaged by a magnetic resonance apparatus such that the localization of the device in the object may be known, and which overcomes the shortcomings of conventional invasive devices, e.g., a catheter.
It is another object of the present invention to provide a magnetic resonance imaging system which includes a magnetic resonance imaging apparatus which is constructed to operate with an RF-safe invasive device capable of being localized in an object being imaged without causing undue uncomfortability in the imaged object resulting from heating during imaging by the magnetic resonance imaging apparatus.
It is yet another object of the invention to provide an RF-safe invasive device equipped with at least one long conductor, for example, a wire loop, for introduction into an object being imaged by a magnetic resonance apparatus such that heating of the conductor and the surrounding tissue during RF transmission is avoided.
It is another object of the invention to provide an RF-safe invasive device which includes a wire loop which is divided into electrical sections in order to minimize induced current flowing in the wire loop when RF-transmission is xe2x80x98ONxe2x80x99.
It is another object of the invention to provide an RF-safe invasive device which includes an electrical connection used for transmitting RF from the distal end to the proximal for localization or imaging, which connection is effectively divided into sections in order to avoid induced current flowing in the electrical connection when RF-transmission is xe2x80x98ONxe2x80x99.
As mentioned above, the RF-safe invasive device of this invention operates with a magnetic resonance apparatus for imaging an object with the introduced invasive device. The magnetic resonance apparatus includes a first magnet system for generating a static magnetic field in the object, a plurality of gradient coils for generating gradient magnetic fields in the object, at least one RF coil for transmitting RF pulses to the object and for receiving RF signals from the object, and a control unit for controlling the gradient magnetic fields and the RF pulses in order to generate magnetic resonance imaging signals.
The introduced invasive device includes an elongate envelope constructed to have a proximal end and a distal end, where at least one long conductor is arranged within the elongate envelope for a specific purpose. For example, for carrying a direct current for catheter localization, such as disclosed in the ""362 patent, or for imaging and/or localization as disclosed in the ""025 patent, transmitting a signal generated by a sensor, etc.
Into each long conductor at least one series element of controllable impedance is incorporated which can effectively divide the long conductor into short electrical sections at times in which RF signals are generated by the magnetic resonant imaging apparatus. The controllable series element may be included to avoid formation of standing waves in the long conductor, and to therefore minimize heating in the long conductor and consequential uncomfortability in the object being imaged.
In a second embodiment, the RF-safe invasive device of this invention may be introduced into an object being imaged with a magnetic resonance imaging apparatus, the localization of the device in the object being monitored during examination by the MRI apparatus. The invasive device includes an elongate envelope constructed with a proximal end and a distal end, a wire loop arranged within the elongate envelope for generating a locating signal such as that described in the ""362 patent and an electric connection for coupling the wire loop to a control unit.
The wire loop includes at least one circuit element which is activated by a control unit at times during which RF signals are generated by the magnetic resonant imaging apparatus in order to avoid heating in the wire loop and consequential uncomfortability in the object being imaged. The loop generates the locating signal during a time in which the control unit causes DC to be provided to the wire loop.
In a third embodiment, the invention comprises a magnetic resonance system (for imaging an object) constructed for operation with an introduced RF-safe invasive device such that the localization of the invasive device may be determined. The magnetic resonance system includes a magnetic resonance apparatus and an RF-safe invasive device constructed in accordance with the principles disclosed herein. The magnetic resonance apparatus may be any MRI device known to those skilled in the art to include a first magnet system for generating a static magnetic field in the object to be imaged, a plurality of gradient coils for generating gradient magnetic fields in the object, at least one RF coil for transmitting RF pulses to the object and for receiving RF signals from the imaged object, and a control unit for controlling the gradient magnetic fields and the RF pulses in order to generate magnetic resonance imaging signals.
The RF-safe invasive device includes an elongate envelope constructed with a proximal end and a distal end, a wire loop arranged within the elongate envelope for generating a locating signal, and an electric connection for coupling the wire loop to a control unit. The wire loop includes at least one circuit element which is activated by the control unit at times in which RF signals are generated by the magnetic resonant imaging apparatus to avoid formation of standing waves in the wire loop and consequential heating in the wire loop leading to uncomfortability in the object being imaged as a result of the heating. The loop generates the locating signal during a time in which the control unit causes DC to be provided to the wire loop.